Gas-Assisted Embossing and UV Curing Combine to Make Microlens Arrays

Individual microlenses may be small, but a large array of them can have a big impact — and present some big challenges. Because microlens arrays are a key optical component in large-format LCD TVs and elsewhere, there is a need for low-cost and high-throughput manufacturing methods. As a variety of techniques have been tried, the challenges of each have become apparent. The least expensive methods suffer from control problems, while those offering the best control lack throughput or are costly.

Researchers formed a microlens array using a method that integrated gas-assisted embossing and UV curing into one process. Optical microscope images of microlenses in the array on a PMMA substrate are shown in a and b, and their height profile is in c. Images reprinted with permission from Optics Express.Now researchers from National Taiwan University in Taipei and from Industrial Technology Research Institute in Hsinchu, also in Taiwan, have arrived at what may be a solution to the problem by combining two processes. They integrated gas-assisted embossing and UV-curing processes into one, thereby avoiding delay-inducing high temperatures and stress-inducing high pressure. The technique addresses the problems of nonuniform pressure and residual stress, two factors that previously had limited microlens uniformity and performance.

In a demonstration of the fabrication process, the scientists used an 0.8-mm-thick stainless steel stamper measuring 270 × 195 mm. Supplied by the Industrial Technology Research Institute, the stamper had an array of more than 1500 × 1500 holes, each with an average diameter of 120.5 μm.

The researchers measured the optical performance of the lenses with a laser and camera in an optical setup that generates a light spot for each lens (left). The uniformity of the light pattern shows that the microlenses are uniform (right).The researchers coated the stamper with a UV-curable liquid resin, fixed it in a holder in a processing chamber, then sealed the chamber and brought a substrate of the polymer PMMA into contact with the stamper. After lowering the pressure on the bottom of the stamper and raising it on the top, they pressed the stamper and substrate together for a fixed time. With the liquid resin transferred onto the substrate, they cured it with UV irradiation at room temperature. Subsequently, they equalized the pressure and removed the stamper from the substrate, leaving behind a microlens array on the substrate’s surface.

After fabricating a microlens array larger than 1500 × 1500, the investigators measured the surface profile of 30 microlenses that had been randomly selected from each of nine areas. They found the average diameter to be 119.77 μm, with an average sag height of 7.89 μm. The holes were not as uniform as expected, which happened because of nonuniformities in the stamper. Changes in its manufacturing process would improve these results, the researchers noted.

They tested the optical properties of the microlenses in the array, sending a helium-neon laser beam through an expanding lens so that the entire array was illuminated. They then captured the images using optics and a CCD, finding the average focal length to be 418 μm. This result was as predicted, given the microlens parameters. The light spots formed by the array were uniform, indicating that the microlenses themselves were uniform.

Tests performed with a strain viewer from Sharples Stress Engineers Ltd. of Preston, UK, showed no photoelastic stress and negligible residual stress. Together with the uniformity and optical performance, such results indicate that the process has potential for large-area fabrication of microstructures, the researchers said.